1. Field of the Invention
The present invention pertains to an integrated circuit arrangement, in particular, in-accordance with the CMOS technology, with a transconductance amplifier.
Transconductance amplifiers that are also referred to as xe2x80x9cOperational Transconductance Amplifiersxe2x80x9d (OTA), transconductance elements or transconductors are devices for generating a current signal from an input voltage signal. The following applies if the input voltage is symbolized by vin and the output current is symbolized by iout:
iout=Gmxc3x97vin
wherein Gm symbolizes the so-called transconductance gain or simply the transconductance of the device.
2. Description of the Prior the Art
A non-linearity of the transconductance amplifier, at which the transconductance depends on the input voltage, frequently causes problems in practical applications. Transconductance amplifiers may, similar to operational amplifiers (op amps), be operated in the feedback mode in order to decisively define the overall characteristics of the device by the external feedback wiring and to largely compensate this non-linearity. The reduction of the useful signal bandwidth associated with a feedback is, in particular, unacceptable for numerous applications.
A highly linear transconductance is achieved with a transconductance amplifier, the transconductance of which is essentially defined by an ohmic resistance, wherein the input voltage is applied to this resistance and the current flowing through the resistance defines the output current. A transconductance amplifier of this type is, for example, known from U.S. Pat. No. 5,451,901. However, it is disadvantageous in numerous applications that the highly linear transconductance defined by the resistance cannot be varied in such a xe2x80x9cresistance-basedxe2x80x9d transconductance device. It would, for example, be conceivable to connect additional resistances in parallel by means of transistors in a device of this type. However, the transistors used lead to parasitic capacitances and an increased non-linearity or to a reduction of the maximum attainable input voltage excursion at a predetermined linearity requirement.
In the practical utilization of transconductance amplifiers in integrated circuit arrangements, one also needs to take into account that transconductances can vary significantly due to fluctuations in the manufacturing process, with the transconductances also varying with the temperature during the operation of the circuit arrangement. These are the reasons why an optional trimming or adjustment of transconductances (or an optional compensation of transconductance changes) is desirable and frequently even necessary.
In order to adjust the transconductance, it is, for example, possible to utilize a transconductance device, the transconductance of which is essentially defined by a transistor arrangement, wherein the transformation of the voltage signal into a current signal is realized by means of transistors (bipolar or MOS). In this case, the inherent transconductances of the transistors are utilized, wherein these transconductances are adjusted in the form of a corresponding shift of the operating point. Transconductance amplifiers of this type are also known from initially cited U.S. Pat. No. 5,451,901. The relatively low linearity of such a xe2x80x9ctransistor-basedxe2x80x9d device is disadvantageous or unacceptable in numerous applications.
A parallel connection of several controlled, identically configured transconductance stages is known from German patent DE 690 26 858 T2 (translation of European patent EP 0 388 802 B1). In this case, the inputs of the transconductance stages are connected to one another, and the outputs of the transconductance stages are connected to one another. The parallel connection serves for increasing the current capacity on the output side.
The present invention is based on the objective of eliminating the above-mentioned disadvantages and, in particular, developing a transconductance amplifier, as well as integrated circuit arrangements provided with such a transconductance amplifier, in which the transconductance is highly linear, but still adjustable.
This objective is attained with an integrated circuit arrangement according to claim 1. The dependent claims pertain to advantageous additional developments of the invention.
The basic idea of the invention consists of combining the high linearity achieved with resistance-based transconductance devices with the adjustability achieved with transistor-based devices. According to the invention, this combination is realized in the form of a parallel connection of a resistance-based transconductance stage and a transistor-based transconductance stage. In order to advantageously limit the non-linearity associated with the utilization of the transistor-based transconductance stage, the invention also proposes that the resistance-based transconductance is higher than the transistor-based transconductance, preferably by at least a factor of 2, in particular, by at least a factor of 5.
Consequently, the invention provides an integrated circuit arrangement with a transconductance amplifier, the transconductance of which is highly linear and simultaneously adjustable. This means that a conflict (linearity and adjustability) can be easily solved such that the performance characteristics of integrated circuit arrangements can be drastically improved in numerous applications.
A transconductance amplifier that is realized as described above is, according to one preferred application, utilized in a continuous-time active filter, e.g., a so-called Gm/C filter, in which a capacitor stage for integrating the output signal is arranged at the output of the transconductance amplifier, wherein said capacitor stage consists, for example, of two capacitors that respectively connect one of two output terminals of the transconductance amplifier to a reference potential (e.g., the ground or a supply potential).
According to one preferred and even more specialized application, the invention is utilized in continuous-time sigma-delta analog/digital converters (ADCs) that, according to the invention, may be realized for high signal bandwidths with a high signal-to-noise ratio, namely in a fully integrated fashion in accordance with the CMOS technology. In such ADCs, the filter characteristics may, for example, be roughly tuned by correspondingly switching on or off integration capacitors (trimming). In this case, the fine-tuning may be realized by adjusting the transistor-based transconductance, in particular, by adjusting at least one current flowing through a transistor channel (in bipolar transistors: emitter-collector path). In order to prevent an excessively large number of rough-tuning stages for adjustable filters, the fine-tuning range should not be excessively small. In applications that are particularly interesting for the invention, this can be achieved if the nominal transconductance (e.g., an average value of the minimum and the maximum transconductance) of the transistor-based transconductance stage is lower than the transconductance of the resistance-based transconductance stage by no more than a factor of 100.
In an integrated circuit arrangement with such a filter, an adjusting device for the automatic chip-integrated tuning of filter characteristics is preferably provided. Such devices and methods for tuning a filter are generally known and may, according to the invention, be advantageously utilized for adjusting the transconductance. In one such method, the current filter performance characteristics are, for example, measured and then compared with a standard (reference). Subsequently, the deviation between the current performance characteristics and the reference is determined, and a correction signal is ultimately calculated and fed to the filter in order to reduce the deviation (error), wherein this method is carried out iteratively. This method may be carried out directly on the filter or indirectly on a replica of the filter or parts of the filter in order to obtain filter characteristics to be measured during the adjustment of the filter.
According to one embodiment, a fully differential, in particular, symmetrically configured transconductance amplifier is provided as disclosed in claim 2.
A particularly simple configuration of a transconductance stage is achieved with the characteristics of claim 3. If the resistance-based transconductance stage and the transistor-based transconductance stage are realized in this fashion, parallel current sources naturally may be combined, i.e., considered as and realized in the form of a single current source, due to the parallel connection of these transconductance stages. If the resistance-based transconductance stage is realized in this fashion, the transistors on the input side may, in particular, be arranged as described in claim 4 and/or the transistors on the input side may be operated as described in claim 5.
With respect to a particularly simple design of the transistor-based transconductance stage, as well as its adjustability, it is advantageous to take the measures disclosed in claims 6 and 7, respectively. In this case, it is preferred to realize the transistors on the input side, as well as the transistor pair, in the form of MOS-FETs.
The current sources of the transconductance amplifier may be simply realized in accordance with claim 8, wherein a cascode transistor may be provided for increasing the output impedance. If a pair of sources is used, it is possible to provide a fully differential operational amplifier in this case in order to improve the performance characteristics of the current sources (xe2x80x9cgain-boostingxe2x80x9d).